1. Field of the Invention
The present invention relates to a lens barrel having an optical element which is deformed by its own weight and/or by being held in position and also to a projection aligner having the lens barrel. More particularly, the present invention relates to a lens barrel having, as such an optical element, for example, a binary-type diffractive optical element and to a projection aligner having the lens barrel.
2. Description of Related Art
Optical systems of the kind having diffractive optical elements have been deveoped in various manners during recent years. The diffractive optical elements known to be used for the optical systems include, for example, Fresnel zone plates, kinoforms, binary optics and hologram elements.
The diffractive optical element is used for converting an incident wavefront into a predetermined wavefront and has features not possessed by a refraction-type lens. For example, the diffractive optical element has dispersion reverse to that of the refraction-type lens and can be formed thin, so that the whole optical system can be compactly constructed.
Generally, with the diffractive optical element arranged to be in a binary type shape, the diffractive optical element can be manufactured by using the manufacturing technique for semiconductor devices. With such a manufacturing technique applied, the diffractive optical element can be manufactured without difficulty to have fine pitches. In view of this, researches are being actively conducted for a binary-type diffractive optical element of the blazed shape which approximates to a stepped shape.
Meanwhile, various methods have been employed for positioning optical elements, such as a diffractive optical element, a lens, etc., within a lens barrel. The known methods include a lens pressing method, a throw-in method, etc.
FIG. 1 shows in outline the structural arrangement of a lens barrel in which optical elements are positioned by the lens pressing method. Referring to FIG. 1, lenses 8 are arranged to constitute a projection optical system. The lens barrel 9 is arranged to hold the lenses 8. Retaining rings 10 are arranged in the lens barrel 9 to fix the lenses 8 to their positions by causing the lenses 8 to abut on the respective lens setting parts "a" of the lens barrel 9.
The outside shapes of the lenses 8 are beforehand cut to be coaxial with respect to a lens optical axis La by machining to a predetermined degree of precision, and the outside diameters of them are beforehand measured and determined also to a predetermined degree of precision.
The inside diameter of the lens barrel 9 is beforehand cut and determined according to the outside diameters of the lenses 8 measured, in such a way as to have a predetermined clearance between the inside diameter of the lens barrel 9 and the outside diameter of each of the lenses 8 when the lenses 8 are fitted in the lens barrel 9.
The lenses 8 are positioned in the direction of the optical axis La by screwing a male screw part formed on the peripheral part of each of the retaining rings 10 into the corresponding one of female screw parts 90 formed in the inner side wall of the lens barrel 9. Each of the retaining rings 10 is thus screwed to cause each of the lenses 8 to abut on the corresponding lens setting part "a", so that the lenses 8 are fixed in position.
In the case of the conventional lens barrel shown in FIG. 1, since each of the lenses 8 is pushed against the corresponding lens setting part "a", the surface shape of each of the lenses 8 tends to be deformed according to the shape of the retaining ring 10 and the shape of the lens setting part "a". Such deformation has presented such a problem as to cause the optical characteristics of the lenses 8 to vary.
To solve the above problem, it is possible to lessen a pushing force on the lens 8, by sticking the lens 8 to the inner wall of the lens barrel 9 by adhesives without using the retaining ring 10. However, in a case where the direction of the optical axis coincides with the direction of gravitation, the lens 8 might be sometimes deformed by its own weight to some extent and in some directions according to the shape of the lens setting part a
It is difficult to machine the lens setting part with its flatness kept more accurate than the flatness of the lens surface. It is also difficult to accurately presume the deformation of the lens abutting on the lens setting part beforehand, because the shape of the lens setting part in each of lens barrels differs from that in another lens barrel. Therefore, in the case of an optical system where even a minute amount of deformation is considered to be a serious drawback, the optical performance of the optical system must be evaluated after assembly work of the optical system and the posture or position of each lens must be adjusted according to the result of the evaluation in a prescribed manner to correct various aberrations resulting from the surface deformation. Accordingly, the number of necessary assembly and adjustment processes has been increased by such additional processes that are necessary.
Further, in a case where a thin optical element such as a diffractive optical element or the like is to be held by a lens barrel, in particular, the amount of the above-stated deformation becomes too much to obtain a desired optical performance by adjusting the posture or position of the optical element.